When feeding a web of flexible non-metallic material, such as an elongate sheet of paper, plastic, fabric, etc., for various purposes, such as to apply a coating thereto, to cure a coating thereon, to print thereon, etc., the web is pulled from a supply or source, such as a storage drum or reel or some other supply source, or is otherwise moved along a path. Usually one or more rolls or the like support the web as it is moved along a path and one or more drive rolls (also referred to as a "pull roll") pull the web along that path. Often it is desirable to provide relatively strong engagement or frictional engagement of the web and the surface of a drive roll to avoid slippage therebetween. Such slippage can result in non-uniform speed of web movement along the path, and such non-uniformity may detrimentally affect the web, the coating or the like applied to the web, etc. For example, such non-uniform speed can result in non-uniform coating of the web, non-uniform curing of the coating material, wrinkles, folds, and/or tears in the web, and so on.
Therefore, often it is desirable to maintain a controlled or adjustable uniform speed of travel of a web through coating equipment, heating or curing equipment, and/or other equipment. However, the speed of travel may tend to change as the diameter of the supply reel and/or the take up drum or reel changes when the web is transferred from one to the other. As those changes occur, the force required to maintain tension on the web and/or to maintain the pulling force on the web may vary. Various techniques have been used to hold the web to a pull roll used to move the web along the path. One technique is to use one roll, such as an idler roll, to press the web against the pull roll. The idler roll may be located in direct confronting engagement with the pull roll separated from it only by the web or the idler roll may be relatively upstream or downstream of the pull roll to urge the web into engagement with the pull roll; in either case the idler roll usually engages a surface of the web opposite the surface which is engaged by the pull roll. There are several disadvantages to using such idler roll technique, two of which are, as follows: the idler rolls add to the size, expense, and mechanical servicing requirements of the equipment; and sometimes it is especially undesirable for a roll surface, such as that of an idler roll, to engage the mentioned "opposite" surface of the web, for example, such surface being one on which a coating or the like has been applied and has not yet cured. The engagement of the idler roll with such a coated surface may damage the coating and/or may result in damaging of the idler roll, for example, in the event coating material were to stick to the idler roll itself.
Idler rolls and/or drive rolls also have been used to form a hard nip between two rolls. A hard nip tends to isolate the relatively upstream and downstream portions of the web relative to each other for various purposes. However, use of more rolls for a hard nip also encounters several of the disadvantages mentioned above. For example, it has been found in one prior web processing machine, to obtain a hard nip for isolating portions of a web moving through the machine between a drive roll and a nip forming roll, such as an idler roll, can require approximately 3 to 5 horsepower (2,100-3,500 watts) energy to move the drive roll of such a hard nip. It would be desirable to be able to provide a hard nip without requiring such large amount of energy.
A vacuum technique also has been used in the past to hold a web to a pull roll. For example, the pull roll may have openings in the surface, and those openings are connected to a vacuum source. The vacuum at those openings draws the web to the pull roll. However, such vacuum techniques are complex and expensive.
An electrostatic technique also has been used in the past to provide relatively strong engagement between a web and the surface of a pull roll without the need to touch the surface of the web opposite the surface which is engaged with the pull roll. An example is described in U.S. Pat. No. Re. 26,951, the entire disclosure of which hereby is incorporated by reference.
Using such an electrostatic technique, a charge of static electricity is applied to the web as it travels along its path, and that charge causes a force which urges the web against the surface of the drive roll. In one such system the drive roll is electrically conductive and a corona discharge device spaced apart from the drive roll provides a static electricity discharge toward the web and drive roll urging the web against the drive roll. An electrostatic system also has been used in the past to apply tension to a web by using the electrostatic force to urge the web against an electrically conductive brake bar; by varying the strength of the electric field produced between the source of electrostatic energy and the brake bar, for example, a drag force of correspondingly varying magnitude can be applied to the web.
In prior electrostatic systems of the type mentioned above the actual force applied to the web varies as various parameters change. The electrostatic derived force applied to the web depends on the current which flows between the electrostatic charge-supplying device and the drive roll or brake bar; and as resistance changes, the current also may change. Such resistance changes may occur due to changes in the gap or spacing between the static charge supplying device and the electrically conductive drive roll or brake bar, for example; and such resistance changes also may be due to changes in ambient humidity, moisture content of the web, composition of the web, thickness of the web, undulations in the web, coating material on the web, etc. The variation in current and, thus, force with which the web is urged into engagement with the drive roll or brake bar can result in slippage of the web relative to the drive roll, change in tension, change in time that a web is located in heating or other curing equipment, change in thickness or amount of coating applied to a surface of the web, etc., each of which can reduce the quality of the finished web product.
Sometimes a coated web is directed through a heated area, such as an oven, in which elevated temperature tends to cure the coating. It is desirable to maintain the uniform speed of a web during coating and curing, for if the web remains too long in the oven, the coating may be non-uniformly cured and/or it or the web itself may be burned. If the web is not in the curing zone of the oven, the coating may not sufficiently cure. These curing problems also may occur when means other than heat is used to induce or to assist curing.
Usually it is desirable to provide uniformity in the distribution of coating material on a surface of a web. The coating may be applied, for example, by a roll which picks up the coating material from a reservoir and applies the coating material to the web surface. However, non-uniformity of the coating can be caused by an imperfection in the application roll, dirt in the reservoir supply, and/or irregularities in the web surface to which the coating is to be applied. The non-uniformity may be due to placing of coating at only some, but not all locations on the web or due to roughness in the coating or uneven thickness or distribution of the coating. Accordingly, there is a need in the art to improve the uniformity in the coating applied to a surface of a web or other sheet material, especially when that web or sheet material is moving continuously.
In accordance with the invention, then, one aspect relates to a method of applying an electrostatic force to a moving web, comprising moving a web in a space between at least one pair of electrodes, supplying a voltage to the electrodes to cause a flow of electrons in the space between the electrodes to apply an electrostatic force to the web, and controlling the current flowing between the electrodes thereby to control the electrostatic force.
Another aspect relates to a method for applying a controlled force to a web, comprising directing an electric current between a source of electrostatic energy and an electrically conductive member to create an electrostatic force to urge a web toward such member, and controlling the current of such source to maintain a desired electric current in said directing step thereby to control the force applied to such web.
A further aspect relates to apparatus for applying electron wind to a material, comprising a plurality of electrodes having a space therebetween, means for supplying current to the electrodes to cause an electron wind in the space, and means for controlling the current to maintain the electron wind substantially constant as the electrical impedance in the space may vary.
An additional aspect relates to a method for applying a controlled corona wind to a material for maintaining a constant force on the material, comprising directing a corona wind toward the material, and controlling the current of the corona wind to maintain a substantially constant current of the corona wind thereby to maintain a desired substantially constant force on the material although the electrical impedance in the path of the corona wind may vary.
Even another aspect relates to a method of controlling tension in a web travelling along a path, comprising directing a corona wind toward web to urge the web against a surface, and adjusting the current flowing in the corona wind thereby to control force urging the web against such surface.
Even a further aspect relates to a method of spreading or smoothing a coating located on a surface, comprising applying a corona wind to the surface with sufficient force to distribute the coating on the surface.
Even an additional aspect relates to a method of avoiding distortions in a moving web, comprising applying an electrostatic force to a web to urge it into engagement with another surface to resist movement and, thereby, to create a tension in the web, and controlling the force with which the web is urged into engagement thereby to control the tension and to maintain the length characteristics of the web substantially constant over the width thereof.
Yet another aspect relates to a method of removing curl from a paper-like web of material travelling along a path, comprising applying moisture to the web, and stretching the web between a drive roll and a relatively hard nip, and forming the relatively hard nip by applying an electrostatic force between a source of electrons and an electrically conductive member to urge the web toward such member.
Yet a further aspect relates to a method of controlling the dimensionality of a web travelling along a path, comprising stretching the web between a drive roll and a relatively hard nip, and forming the relatively hard nip by applying an electrostatic force between a source of electrons and an electrically conductive member to urge the web toward such member.
Yet an additional aspect relates to a method of curing a coating on a web, comprising applying a controlled electrostatic energy field to the web and coating to effect curing of the coating.
To the accomplishment of the foregoing and related ends, the invention, then, comprises the features hereinafter fully described and particularly pointed out in the claims. The following description and the annexed drawings set forth in detail certain illustrative embodiments of the invention. These embodiments are indicative, however, of but several of the various ways in which the principles of the invention may be employed.
Although the invention is shown and described with respect to the embodiments below, it is obvious that equivalents and modifications will occur to others skilled in the art upon the reading and understanding of the specification. The present invention includes all such equivalents and modifications, and is limited only by the scope of the claims.